1. Field of the Invention
The present invention relates to a technique of a mobile communication device, and more particularly, to a mobile communication device with a positioning capability.
2. Description of the Prior Art
Mobile communication devices (such as mobile phones) and global navigation satellite system (GNSS) receivers (such as GPS receivers) are both widely used electronic devices. For many, both of these devices are necessary in daily life. In order to satisfy the users' requirement, integrating GNSS receiver and mobile communication device functions together is becoming a trend. However, when integrating these functions together, many problems must be considered, such as power consumption, hardware cost, circuit board area, etc.
It is well known that both GNSS receivers and mobile communication devices need an oscillator to be a reference frequency source during operation. In prior art, the oscillator utilized by the GNSS receiver is usually an oscillator with high precision, such as a Temperature Compensated Crystal Oscillator (TCXO), which is calibrated to a specific frequency (such as 16.368 MHz). Most oscillators utilized in the mobile communication devices, however, are oscillators with lower precision, such as a Voltage-Controlled Temperature Compensated Crystal Oscillator (VCTCXO).
In order to reduce the hardware cost of integrating GNSS receiver and mobile communication functions together, a U.S. Pat. No. 6,724,342 provides a mobile communication device with a positioning capability, wherein the communication circuit and the positioning signal receiver share the same oscillator. However, the positioning signal receiver is quite sensitive to the precision and frequency drift of the reference frequency output by the oscillator. In the mobile communication device disclosed by the U.S. Pat. No. 6,724,342, if the communication circuit adjusts the output frequency of the shared oscillator while the positioning signal receiver extracts the satellite signals, the positioning signal receiver will not be immediately aware of the change in the oscillator's output frequency, thus resulting in occurrences of positioning errors (such as a positioning location suddenly diverging from a previous positioning location by a wide margin), or even not being able to detect the satellite signals.
One solution for the above problem is to control the shared oscillator to maintain a constant output frequency when the positioning signal receiver is extracting the satellite positioning signals. Unfortunately, this solution will result in a largely increased call drop rate for the mobile communication device, and thus will bring down the whole communication quality.